Some customers have asked why their propane tank gauge never reads 100% after a delivery. Here’s why:
The propane in your tank is a liquid, which changes to gas before it leaves the tank. But like all liquids, it will expand when its temperature rises. To make sure there’s room for the propane to expand safely, we typically fill tanks to about 80% of capacity. So a 500-gallon tank that’s 80% full will safely hold about 400 gallons of propane.
Keep in mind, the amount of propane in your tank doesn’t change when it expands and contracts — just its density.
We call this the 80/20 rule, and it’s especially important in hot weather and with aboveground tanks, which are not insulated from heat the way an underground tank is. Propane gas expansion is also the reason that you should never paint your propane tank a dark color: Dark colors absorb more heat.